Many turbomachine functions must be carried out using friction products with a low coefficient of friction in zones in which conventional lubrication is impossible (conventional lubrication which combines porosity of the material in question and the use of an oil and/or grease). A substantial part of those functions is carried out by small mechanical parts with shapes of greater or lesser complexity (such as bushings in which spindles with variable pitch blades pivot), which parts are subjected to high temperatures and stresses.
Organic materials are used in the “low” temperature range (below 300° C.) and in the “high” temperature range (over 600° C.), metallic materials are used, primarily cobalt-based (if low coefficients of expansion are a secondary consideration), or ceramic materials are used. In the intermediate range, the medium to high temperature range, only carbons are used; their fragility substantially limits their use.